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Archaean magmatism NB- Archean (US spelling) or Archaean (UK spelling)

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Presentation on theme: "Archaean magmatism NB- Archean (US spelling) or Archaean (UK spelling)"— Presentation transcript:

1 Archaean magmatism NB- Archean (US spelling) or Archaean (UK spelling)

2 Why? Somehow different from modern magmas  Interesting to test our understanding of petrogenetic processes Not that rare, and good South African examples (Barberton) Economic interest Gold (large part of world’s gold + secondary deposits) PGE bearing sulphides Nickel Department’s research interests

3 Barberton gold fields

4 Two characteristic rock types Komatiites = ultra-mafic, Mg-rich lavas TTGs = Tonalites, Trondhejmites & Granodiorites Link with Archaean geodynamic style?

5 The Archaean

6 Oldest crustal remnants

7 Jack Hill (Australia) zircon =  Ga

8 The oceanic crust is young…

9 75 % of the crust was formed at ca. 2.5 Ga The Archaean is a major crust-forming period

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11 The average continental crust C.C. is roughly andesitic

12 Continental crust Ca. 30 km

13 The continental crust … has the composition of magmatic rocks, and is largely made of magmatic rocks … formed mostly in the Archaean ► We have to study the geology of Archaean continental crust.

14 Earth’s heat production ►A 2- to 4-fold decrease from the Archaean to now

15 Effects of higher Archaean heat production? Shape of convection Partitioning of heat flux Effects on the continents thermal structure and behaviour Petrogenesis?

16 Shape of convection ? (R a = ) (R a > 10 5 ) Ra = function of many things, including  T (or heat production)

17 Zimbabwe (2.7 Ga) Pilbara (3.5 Ga) Archaean dome-and-keel patterns Vertical tectonics (“sagduction”)

18 Superior Province

19 Bimodal Archaean terranes Greenstone belts (commonly dominated by greenschist facies amphibolites) –Mafic and ultramafic (= komatiites) lavas Some intermediate lavas (andesites) –Detrical sediments Some chemical sediments (BIFs) or biogenic formations (stromatholites) Gneissic « basement » or plutons Late plutons

20 2.9 – 2.7 Ga granites 3.1 Ga granites & syenites Moodies Fig Tree Onverwacht Ca. 3.2 Ga TTG Ca. 3.4 Ga TTG « Ancient gneisses » (3.6 – 3.4 Ga)

21 1. Komatiites Viljoen, M. J. and R. P. Viljoen (1969). "The geology and geochemistry of the lower ultramafic unit of the Onverwacht group and a proposed new class of igneous rocks." Geological Society of South Africa Special Publication 2: A truly South-African rock type!

22 Onverwacht group, BGB The original komatiites in Komatii formation (~1.5 km from type locality)

23 Komatiites composition

24 Structure of komatiites flows Origin of komatiites Komatiites and the Archaean mantle

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26 B4 Polysutured top Random spinifex Orientated spinifex parallel blades of spinifex solid subhedral olivine Basal chill, polysutured Subdivision of komatiite flows (Arndt et al. 1977)

27 Chilled/brecciated top Subaquatic emplacement

28 Spinifex textured layer(s) Spinifex grass, Western Australia (Barnes 1990) Random spinifex Orientated spinifex Plate spinifex

29 Random spinifex

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33 Orientated spinifex

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35 Plate spinifex

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37 Polyedral olivine

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43 Origin of komatiites High Mg contents require high degree of mantle melting (40-60 %) This implies very high temperatures and fast rise

44 What are the implications of komatiites? Probably formed in hot-spot like situations (difficult to arrive to > 1600° else) Even though, this is hotted than modern hotspots At least some parts of the Earth were very hot At least part of the GSB formed from hotspots (intraplate situation)

45 Komatiites and the history of the Archaean mantle

46 3 groups of komatiites, from the shape of their HREE pattern (or Gd/Yb ratios) Role of garnet

47 Correlation with Al (and also Ca) –Al depleted (grp II) vs. Al-undepleted (grp. III) Only grp I komatiites exist in the late Archaean

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49 1.Early differenciation of the Earth mantle (completed at 3.80 Ga) 2.Deep origin of Late-Archaean komatiites (or locally non- differenciated bits of mantle?) Maybe due to a cooler Earth, hot temperatures found only very deep?

50 2. TTG Archaean TTG (Tonalite, Trondhjemites and Granodiorites) ≈ grey gneisses (although in details, some TTGs are not grey gneisses and some grey gneisses are not TTG…)

51 Archaean grey gneisses Some relatively simple orthogneisses Stolzburg pluton (Barberton, 3.45 Ga)

52 Commonly complex, migmatitic, polydeformed orthogneisses

53 The Sand River Gneisses Ca. 3.1 Ga TTG gneisses in Messina area, Limpopo Belt, South Africa (R. White, Melbourne, for scale)

54 However, the most common component of the grey gneisses is relatively constant

55 Mineralogy

56 Major elements

57 REE

58 Nb-Ta anomaly Sr contents Y & HREE depletion

59 Experimental studies

60 Partial melting of amphibolites (= metabasalts) is appropriate to generate TTG-like sodic melts Melting reactions of the form –Amp + Plag = M + Opx + Ilm –Amp + Plag = M + Grt + Ilm (Incongruent melting / amphibole dehydration melting)

61 Conditions for making TTGs Experimental melts In Garnet stability field (Gt in residue) Melting of hydrous basalt KDKD Gt/melt = (other minerals ≤ 1) Yb

62 NB Some people propose that TTGs can be formed by hornblende dominated FC of andesites Not impossible (at least in theory) but.. –Where are the cumulates? –High viscosity of felsic melts –Lack of andesitic plutonic terms associated with TTGs Regarded as unlikely to impossible by maybe 80-90% of the petrologists

63 TTG are... Orthogneisses Tonalites, Trondhjemites & Granodiorites (Na-rich series) Fractionnated REE, etc. Largely homogeneous throughout the Archaean Originated by partial melting of amphibolites (hydrated basalts), in garnet stability field

64 Garnet stability in mafic rocks From a dozen of experimental studies Well- constrained grt- in line at about kbar KDKD Gt/melt = (other minerals ≤ 1) Yb

65 From chemistry to geodynamic TTGs = partial melts of amphibolites in garnet stability field Does this tell something about geodynamic conditions?

66 Geodynamic site ? Thick (oceanic or continental) crust (e.g. Oceanic plateau) Subduction Intermediate cases: Shallow subduction (± underplating) Stacked oceanic crust Gt-in

67 TTGs in a « plate » model

68 TTGs in a « non plate » model

69 Some lines of research TTG and adakites Secular evolution of TTGs TTGs and partial melting of amphibolites Diversity and components of the « grey gneisses » « Sanukitoids » etc. You’re now entering the field of active research and controversies!

70 TTGs and adakites Are TTGs and adakites similar? That’s the stuff active scientific research is made of … Yes ! No !

71 Are TTGs and adakite similar? –If they are: Adakites can be used as an indicator of the site of TTG formation, but… Are the adakites formed as slab melts.. Or as melts of underplated basalts (Cordilera Blanca)? –If they are not: they still are rather similar, so what the… ?

72 Secular evolution of Mg# in TTG Fractional crystallization reduces Mg# For each period the higher Mg# represents TTG parental magma From 4.0 to 2.5 Ga Mg# regularly increased in TTG parental magmas

73 MgO increases inTTG in course of time SiO 2 decreases inTTG in course of time Adakites have exactly the same evolution pattern as (young) TTG For the same SiO 2, experimental melts are systematically MgO poorer than TTG

74 Our conclusions Relatively young TTGs are similar to adakites Both are different from melts from amphibolites (higher Mg etc.) We propose that this corresponds to interactions with the mantle … which can be achieved only in subduction (slab melting) situation – both for young TTGs and adakites Martin & Moyen 2002 NB- This is just our interpretation – it is challenged

75 High heat production  High geothermal gradients  Shallow depth slab melting Thin overlying mantle  No or few magma/mantle interactions  Low Mg-Ni-Cr TTG Lower heat production  Lower geothermal gradients  Deep slab melting Thick overlying mantle  important magma/mantle interactions  High-Mg-Ni-Cr TTGLow heat production  Low geothermal gradients  No slab but mantle wedge melting EARLY ARCHAEAN LATE ARCHAEAN/ADAKITES TODAY INTERPRETATION

76 Sanukitoids: geographic repartition

77 Sanukitoids: petrography Diorites, monzodiorites and granodiorites Lots of microgranular mafic enclaves Qz + Pg + KF + Bt + Hb ± Cpx Ap + Ilm + Sph + Zn

78 Sanukitoids: geochemistry

79 Making sanukitoids

80 Sanukitoids also suggest interactions between the mantle and TTG (or TTG like) melts Again, this is more consistent with slab melting.. At least at the end of the Archaean

81 As usual, the answer is certainly somewhere in between the extremes! Some TTGs are probably slab melts – maybe not all Some TTGs certainly formed in subduction zones (and therefore subduction zones existed quite early) – but probably not all, nor everywhere


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